The last generation of CMB anisotropy experiments
operating either from
space, like the WMAP and Planck satellite, from the atmosphere,
such as balloons, or from the ground, like interferometers, make use of
complex multi-frequency instruments
at the focus of meter class telescopes to allow the
joint study of CMB and foreground anisotropies, necessary
to achieve an accurate component separation.
Between GHz and GHz, where
foreground contamination is minimum, it is extremely
important to reach the best trade-off between
the improvement of the angular resolution, necessary for
measuring the high order acoustic peaks of CMB anisotropy, and the
minimization of the straylight contamination mainly due to the
Galactic emission.
This is one of the most critical systematic effects at large and
intermediate angular scales
(i.e. at multipoles less than 100)
and consists in unwanted radiation entering the beam at large
angles from the direction of the antenna boresight direction.
We consider here the 30 and 100 GHz channels
of the Planck Low Frequency Instrument (LFI).
Assuming the nominal Planck scanning strategy,
we evaluate the straylight contamination
introduced by the most relevant Galactic foreground components
for a reference set of optical configurations, accurately
simulated as described in Sandri et al. ([CITE], A&A, 428, 299) (hereafter Paper I).
Given the overall constraints to the LFI optical design,
we show that it is possible
to improve the angular resolution by % by keeping
the overall peak-to-peak signal of the Galaxy straylight contamination
(GSC) below the level of few μK
(and about 10 times smaller in terms of rms). A comparison between
the level of straylight introduced by the different Galactic components
for different beam regions (intermediate and far beam)
is presented.
We provide approximate relations, both for the intermediate
and the far beam, for the rms and the peak-to-peak
levels of the GSC
as functions of the corresponding contributions to the integrated beam
or of the spillover.
For some reference cases we compare
the results based on Galactic foreground maps derived from
radio, IR, and Hα templates
with those based on WMAP maps including
CMB and extragalactic source fluctuations.
The implications for the GSC in the Planck LFI
polarization data are discussed.
Finally, we compare the results obtained at 100 GHz
with those at 30 GHz, where the GSC is more critical.

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